Now families who prefer to buy smaller boxes of their favourite Christmas chocolates have been warned by consumer campaigners that they are paying up to twice as much as those buying big tubs.

Best-sellers such as Quality Street, Cadbury Heroes and Mars Celebrations have been named in a new report by Which?, which found some supermarkets are charging double per 100g prices for modest sized packs.

For example a 1.32kg tub of Quality Street costs 53p per 100g at Asda, while a 265g box was nearly twice as expensive, at £1.02 per 100g.

Experts accused manufacturers and retailers of becoming "less transparent" on the pricing of goods and said they were not taking seriously their responsibility to make sure Brits consume healthy levels of sugar.

James Daley, director at Fairer Finance, said: "The pricing of groceries is getting worse and less transparent. A very common trend is lowering the amount of goods in a pack and creating a false economy.

"What retailers and manufacturers are doing is not illegal but it affects their reputation and eventually the world will wake up to it and realize they are being stung. Those selling treats such as chocolate also have a responsibility to ensure they are not exacerbating problems like diabetes and this shows they are not necessarily taking it seriously."

All the major supermarkets stock chocolate boxes in different sizes, making it hard to compare like for like, Which said. Some packs weigh 69g with the larger tubs coming in at 1.32kg, Which? found.

Toblerone: not so chunky now

The pricing discrepancy comes after reports of a string of confectionery rip-offs in recent weeks. Last month, Terry's chocolate orange fans were angered after the treat was reduced from 175g to 157g in weight – a cut of 10 per cent.

And last week Toblerone lovers hit out after its makers widened the gaps between the chocolate bar’s distinctive triangular chunks in an effort to avoid putting its prices up.

Which? found Asda had the cheapest pack of both Cadbury Heroes and Mars celebrations at 62p and 58p per 100g respectively. For Nestle Quality Street, Asda and Ocado had the joint-cheapest pack, at 53p per 100g.

Waitrose had the cheapest pack of Cadbury Roses (55p per 100g) and Morrisons had the cheapest Cadbury’s Milk Tray at 93p per 100g, the data showed.

Richard Headland, Which? magazine editor, said: “As the festive shopping season gets into full swing people are starting to stock up on their Christmas treats and snacks.

“Chocolate boxes are a firm favourite and this snapshot research shows that it can be worth comparing prices by weight to save money at this time of year.”